A fluid containing a plurality of compounds, can be analyzed by separating the fluid into different compounds as by HPLC (high pressure liquid chromatography), and by passing each compound through a detector. Within the last few years, mass sensitive detectors such as mass spectrometers, are available, which can detect a small sample (of perhaps 100 nanoliters) in a short period of time such as 20 seconds. The analyzing process can be accomplished more rapidly and in a simpler manner by a mechanism that can take a plurality of different sample flows (where each can come from a different HPLC unit) and flow a small slug of each sample at a time to the detector, with a slug of solvent between the slugs of different samples to identify the beginning and end of each sample slug. Such a mechanism preferably minimizes pressure pulses in the flow of the sample and solvent slugs towards the detector, so a largely constant pressure of fluid is applied to the detector, and so largely constant sample pressure is maintained at the output of each HPLC unit. An apparatus of relatively simple construction and operation which could separate a plurality of sample streams and a solvent stream, to produce a single detector stream of sample slugs separated by solvent slugs, and which minimized pressure pulses along the sample streams and along the detector stream would be of value.